Transistor having source or drain formation assistance regions with improved bottom isolation

ABSTRACT

Embodiments of the invention are directed to a method of performing fabrication operations to form a transistor, wherein the fabrication operations include forming a source or drain (S/D) region having an S/D formation assistance region at least partially within a portion of a substrate. An S/D isolation region is formed around sidewalls and a bottom surface of the S/D formation assistance region and configured to electrically isolate the S/D formation assistance region from the substrate.

BACKGROUND

The present invention relates in general to fabrication methods andresulting structures for semiconductor devices. More specifically, thepresent invention relates to fabrication methods and resultingstructures for transistors having source or drain (S/D) formationassistance regions with improved bottom isolation.

In contemporary semiconductor device fabrication processes, a largenumber of metal oxide semiconductor field effect transistors (MOSFETs),such as n-type field effect transistors (nFETs) and p-type field effecttransistors (pFETs), are fabricated on a single wafer. Non-planar MOSFETarchitectures (e.g., fin-type FETs (FinFETs) and nanosheet FETs) canprovide increased device density and increased performance over planarMOSFETs. For example, nanosheet FETs, in contrast to conventional planarMOSFETs, include a gate stack that wraps around the full perimeter ofmultiple stacked and spaced apart nanosheet channel regions for areduced device footprint and improved control of channel current flow.

In nanosheet FETs, S/D regions can be formed using a variety ofprocesses. A known S/D fabrication process uses exposed end regions ofthe nanosheets as seed (or nucleation) regions from which doped S/Dregions are epitaxially grown.

SUMMARY

Embodiments of the invention are directed to a method of performingfabrication operations to form a transistor, wherein the fabricationoperations include forming a source or drain (S/D) region having an S/Dformation assistance region at least partially within a portion of asubstrate. An S/D isolation region is formed around sidewalls and abottom surface of the S/D formation assistance region and configured toelectrically isolate the S/D formation assistance region from thesubstrate.

Embodiments of the invention are directed to a method of performingfabrication operations to form a transistor, wherein the fabricationoperations include forming over a substrate a first nanosheet having afirst nanosheet end region, wherein a sidewall surface of the firstnanosheet end region defines a first portion of a source or drain (S/D)trench. A second nanosheet having a second nanosheet end region isformed over the first nanosheet, wherein a sidewall surface of thesecond nanosheet end region defines a second portion of the S/D trench.Formed over the substrate is a third nanosheet having a third nanosheetend region, wherein a sidewall surface of the third nanosheet end regiondefines third portion of the S/D trench. Formed over the third nanosheetis a fourth nanosheet having a fourth nanosheet end region, wherein asidewall surface of the fourth nanosheet end region defines a fourthportion of the S/D trench. An S/D formation assistance region is formedwithin a portion of the substrate, wherein a top surface of the S/Dformation assistance region defines a bottom surface of the S/D trench.An S/D region is formed in the S/D trench, wherein forming the S/Dregion includes growing a first portion of the S/D region from thesidewall surface of the first nanosheet end region; growing a secondportion of the S/D region from the sidewall surface of the secondnanosheet end region; growing a third portion of the S/D region from thesidewall surface of the third nanosheet end region; growing a fourthportion of the S/D region from the sidewall surface of the fourthnanosheet end region; and growing a fifth portion of the S/D region fromthe top surface of the S/D formation assistance region. Prior to thefirst portion merging with the third portion, the fifth portion mergeswith the second portion and the third portion to form a first mergedportion of the S/D region.

Embodiments of the invention are directed to a transistor that includesa source or drain (S/D) region and an S/D formation assistance region ofthe S/D region. An isolation region is formed around sidewalls and abottom surface of the S/D formation assistance region and configured toelectrically isolate the S/D formation assistance region from thesubstrate.

Additional features and advantages are realized through techniquesdescribed herein. Other embodiments and aspects are described in detailherein. For a better understanding, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as embodiments is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features and advantages ofthe embodiments are apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings in which:

FIGS. 1-13 depict multiple cross-sectional views of a nanosheet-basedstructure after various fabrication operations for forming a transistorhaving source or drain (S/D) formation assistance regions with improvedbottom isolation in accordance with aspects of the invention, in which:

FIG. 1 depicts cross-sectional views of the nanosheet-based structureafter initial fabrication operations in accordance with aspects of thepresent invention;

FIG. 2 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention;

FIG. 3 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention;

FIG. 4 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention;

FIG. 5 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention;

FIG. 6 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention;

FIG. 7 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention;

FIG. 8 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention; and

FIG. 9 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention;

FIG. 10 depicts a cross-sectional view of the nanosheet-based structureafter fabrication operations in accordance with aspects of the presentinvention;

FIG. 11 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention;

FIG. 12 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention; and

FIG. 13 depicts a cross-sectional view of the nanosheet-based structureafter additional fabrication operations in accordance with aspects ofthe present invention.

DETAILED DESCRIPTION

Although this detailed description includes examples of how aspects ofthe invention can be implemented to form a transistor having S/Dformation assistance regions with improved bottom isolation,implementation of the teachings recited herein are not limited to aparticular type of FET structure or combination of materials. Rather,embodiments of the present invention are capable of being implemented inconjunction with any other type of transistor device (planar,non-planar, p-FET, n-FET, fin-type FET) or material (e.g., Si or SiGe),now known or later developed, wherein it is desirable to provide S/Dformation assistance regions with improved bottom isolation.

For the sake of brevity, conventional techniques related tosemiconductor device and integrated circuit (IC) fabrication may or maynot be described in detail herein. Moreover, the various tasks andprocess steps described herein can be incorporated into a morecomprehensive procedure or process having additional steps orfunctionality not described in detail herein. In particular, varioussteps in the manufacture of semiconductor devices andsemiconductor-based ICs are well known and so, in the interest ofbrevity, many conventional steps will only be mentioned briefly hereinor will be omitted entirely without providing the well-known processdetails.

Turning now to a description of technologies that are more specificallyrelevant to aspects of the present invention, semiconductor devices(e.g., FETs) are formed using active regions of a wafer. The activeregions are defined by isolation regions used to separate andelectrically isolate adjacent semiconductor devices. For example, in anIC having a plurality of MOSFETs, each MOSFET has a source and a drainthat are formed in an active region of a semiconductor layer byimplanting n-type or p-type impurities in the layer of semiconductormaterial. Disposed between the source and the drain is a channel (orbody) region. Disposed above the body region is a gate electrode. Thegate electrode and the body are spaced apart by a gate dielectric layer.

MOSFET-based ICs are fabricated using so-called complementary metaloxide semiconductor (CMOS) fabrication technologies. In general, CMOS isa technology that uses complementary and symmetrical pairs of p-type andn-type MOSFETs to implement logic functions. The channel region connectsthe source and the drain, and electrical current flows through thechannel region from the source to the drain. The electrical current flowis induced in the channel region by a voltage applied at the gateelectrode.

The wafer footprint of an FET is related to the electrical conductivityof the channel material. If the channel material has a relatively highconductivity, the FET can be made with a correspondingly smaller waferfootprint. A known method of increasing channel conductivity anddecreasing FET size is to form the channel as a nanostructure. Forexample, a so-called gate-all-around (GAA) nanosheet FET is a knownarchitecture for providing a relatively small FET footprint by formingthe channel region as a series of thin nanosheets (e.g., about 3 nm toabout 8 nm thick). In a known GAA configuration, a nanosheet-based FETincludes a source region, a drain region and stacked nanosheet channelsbetween the source and drain regions. A gate surrounds the stackednanosheet channels and regulates electron flow through the nanosheetchannels between the source and drain regions.

GAA nanosheet FETs are fabricated by forming alternating layers ofnon-sacrificial nanosheets and sacrificial nanosheets. The sacrificialnanosheets are released from the non-sacrificial nanosheets before theFET device is finalized. For n-type FETs, the non-sacrificial nanosheetsare typically silicon (Si) and the sacrificial nanosheets are typicallysilicon germanium (SiGe). For p-type FETs, the non-sacrificialnanosheets can be SiGe and the sacrificial nanosheets can be Si. In someimplementations, the non-sacrificial nanosheet of a p-type FET can beSiGe or Si, and the sacrificial nanosheets can be Si or SiGe. Formingthe GAA nanosheets from alternating layers of non-sacrificial nanosheetsformed from a first type of semiconductor material (e.g., Si for n-typeFETs, and SiGe for p-type FETs) and sacrificial nanosheets formed from asecond type of semiconductor material (e.g., SiGe for n-type FETs, andSi for p-type FETs) provides superior non-sacrificial electrostaticscontrol, which is necessary for continuously scaling gate lengths downto seven (7) nanometer CMOS technology and below. The use of multiplelayered SiGe/Si sacrificial/non-sacrificial nanosheets (or Si/SiGesacrificial/non-sacrificial nanosheets) to form the channel regions inGAA FET semiconductor devices provides desirable device characteristics,including the introduction of strain at the interface between SiGe andSi.

The S/D regions of the transistor can be formed in S/D trenches by anepitaxial growth process in which separate individual S/D sub-regionsare epitaxially grown from sidewall surfaces at end regions of thenon-sacrificial nanosheets. As the epitaxial growth process continues,the S/D sub-regions merge to form a single S/D region. If the S/Dsub-regions in the upper region of the S/D trench merge before S/Dsub-regions in the bottom region of the S/D trench merge, the merged S/Dregion in the upper region of the S/D trench can cut off the flow ofepitaxial gas/liquid to the S/D sub-regions in the bottom region of theS/D trench, which stops the epitaxial growth process in the lower regionof the S/D trench before the S/D sub-regions in the lower region of theS/D trench merge. The unmerged sub-regions result in no S/D materialbeing present in the bottom region of the S/D trench, which reduces thevolume of S/D material, increases S/D resistance, and increases thetransistor's switching times.

Turning now to an overview of aspects of the invention, embodiments ofthe invention address the above-described shortcomings in known S/Dregion fabrication processes by providing fabrication methods andresulting structures for forming S/D regions in transistors byincorporating S/D formation assistance regions with improved bottomisolation. In embodiments of the invention, alternating layers ofnon-sacrificial nanosheets and sacrificial nanosheets are etched intoadjacent nanosheet stacks, and end regions of the sacrificial nanosheetsin the adjacent nanosheet stacks are replaced with inner spacers formedfrom dielectric material. The space between adjacent nanosheet stacksdefines sidewall portions of an S/D trench in which an S/D region willbe formed. In accordance with aspects of the invention, an S/D formationassistance (or nucleation) region is formed in a portion of thetransistor's substrate, and an exposed surface of the S/D assistanceregion defines a bottom surface of the S/D trench.

In embodiments of the invention, the S/D regions can be formed by angrowth process in which separate individual S/D sub-regions are grown ina substantially lateral (or horizontal) direction from sidewall surfacesat end regions of the non-sacrificial nanosheets, as well as in asubstantially vertical direction from the exposed surface of the S/Dassistance region formed in the transistor's substrate. In embodimentsof the invention, the S/D sub-regions can be epitaxially grown fromgaseous or liquid precursors using, for example, vapor-phase epitaxy(VPE), molecular-beam epitaxy (MBE), liquid-phase epitaxy (LPE), orother suitable process.

Embodiments of the invention control the formation of “cross-trench” S/Dsub-region mergers during formation of S/D regions. The terms“cross-trench” refer to a merged S/D sub-regions that spans across theS/D trench in a manner that blocks epitaxial gas and/or liquid fromflowing to structures below the cross-trench merged S/D sub-regions. Inaccordance with aspects of the invention, the exposed surface of the S/Dassistance region has a lateral width dimension that is larger than avertical height (or thickness) of each of the sidewall surfaces at endregions of the non-sacrificial nanosheets. The width dimension of theexposed surface of the S/D assistance region ensures that the S/Dsub-region that is vertically grown from the exposed surface of the S/Dassistance region will merge with the S/D sub-regions near the bottomregion of the S/D trench that are laterally grown from the sidewallsurfaces at end regions of the non-sacrificial nanosheets before the S/Dsub-regions at an upper regions of the S/D trench merge. Accordingly,gas or liquid flow to the S/D sub-regions in the lower region of the S/Dtrench is not cut off prematurely (i.e., before these lower regionsub-regions merge), thereby ensuring that the epitaxial growth processin both the upper region and the lower region of the S/D trench isallowed to proceed until all of the S/D sub-regions have merged.

In embodiments of the invention, an S/D isolation region is formed inthe transistor's substrate, and the S/D assistance region is formedwithin the S/D isolation region. The S/D isolation region is configuredto electrically isolate the S/D formation assistance region from thesubstrate. In some embodiments of the invention, a first bottomisolation region is positioned between one nanosheet stack and thesubstrate, and a second bottom isolation region is positioned betweenthe adjacent nanosheet stack and the substrate. In some embodiments ofthe invention, the first bottom isolation region is physically coupledto the S/D isolation region, and the second bottom isolation region isphysically coupled to the S/D isolation region. In some embodiments ofthe invention, a vertical depth dimension of the first bottom isolationregion is less than a vertical depth dimension of the S/D isolationregion. In some embodiments of the invention, a vertical depth dimensionof the second bottom isolation region is less than a vertical depthdimension of the S/D isolation region.

Turning now to a more detailed description of fabrication operations andresulting structures according to aspects of the invention, FIGS. 1-13depict a nanosheet-based structure 100 after various fabricationoperations for forming nanosheet FETs having S/D formation assistanceregions with improved bottom isolation. For ease of illustration, thefabrication operations depicted in FIGS. 1-13 will be described in thecontext of forming 1-3 nanosheet stacks 130 (shown in FIG. 1). It isintended, however, that fabrication operations described herein applyequally to the fabrication of any number of the nanosheet stacks 130.

Although the cross-sectional diagrams depicted in FIGS. 1-13 aretwo-dimensional, it is understood that the diagrams depicted in FIGS.1-13 represent three-dimensional structures. To assist with visualizingthe three-dimensional features, the top-down reference diagram 101 shownin FIG. 1 provides a reference point for the various cross-sectionalviews (X-view, Y1-view, and Y2-view) shown in FIGS. 1-13. The X-view isa side view taken across the three gates, the Y1-view is an end viewtaken through the active gate, and the Y2-view is an end view takenthrough a portion of the nanosheet (NS) stack where one of the S/Dregions is (or will be) formed.

FIG. 1 depicts cross-sectional views of the nanosheet-based structure100 after initial fabrication operations in accordance with aspects ofthe present invention. As shown in FIG. 1, the nanosheet stack 130 isformed over the substrate 102. The nanosheet stack 130 includes analternating series of SiGe sacrificial nanosheet layers 120, 122, 124,126 and Si nanosheet layers 114, 116, 118. In accordance with aspects ofthe invention, the alternating nanosheet layers 120, 122, 114, 124, 116,126, 118 of the nanosheet stack 130 are formed by epitaxially growingone nanosheet layer then the next until the desired number and desiredthicknesses of the nanosheet layers are achieved. A hard mask layer (notshown) is deposited over the alternating nanosheet layers 120, 122, 114,124, 116, 126, 118, and the hard mask layer and the alternatingnanosheet layers 120, 122, 114, 124, 116, 126, 118 are etched to definethe hard mask (HM) 128, the nanosheet stack 130, and the sub-fin 102A ofthe substrate 102. The hard mask layer and the resulting HM 128 can beany suitable dielectric, including but not limited to SiN.

In embodiments of the invention, each of the nanosheet layers 120, 122,114, 124, 116, 126, 118 can have a vertical direction thickness in therange from about 5 nm to about 20 nm, in the range from about 10 nm toabout 15 nm, or about 10 nm. Other vertical direction thicknesses arecontemplated. Although eight alternating layers 120, 122, 114, 124, 116,126, 118 are depicted in the figures, any number of alternating layerscan be provided.

Epitaxial materials can be grown from gaseous or liquid precursorsusing, for example, vapor-phase epitaxy (VPE), molecular-beam epitaxy(MBE), liquid-phase epitaxy (LPE), or other suitable process. Epitaxialsilicon, silicon germanium, and/or carbon doped silicon (Si:C) siliconcan be doped during deposition (in-situ doped) by adding dopants, n-typedopants (e.g., phosphorus or arsenic) or p-type dopants (e.g., boron orgallium), depending on the type of transistor.

The terms “epitaxial growth and/or deposition” and “epitaxially formedand/or grown” mean the growth of a semiconductor material (crystallinematerial) on a deposition surface of another semiconductor material(crystalline material), in which the semiconductor material being grown(crystalline overlayer) has substantially the same crystallinecharacteristics as the semiconductor material of the deposition surface(seed material). In an epitaxial deposition process, the chemicalreactants provided by the source gases are controlled and the systemparameters are set so that the depositing atoms arrive at the depositionsurface of the semiconductor substrate with sufficient energy to moveabout on the surface such that the depositing atoms orient themselves tothe crystal arrangement of the atoms of the deposition surface.Therefore, an epitaxially grown semiconductor material has substantiallythe same crystalline characteristics as the deposition surface on whichthe epitaxially grown material is formed. For example, an epitaxiallygrown semiconductor material deposited on a {100} orientated crystallinesurface will take on a {100} orientation. In some embodiments of theinvention, epitaxial growth and/or deposition processes are selective toforming on semiconductor surfaces, and generally do not deposit materialon non-crystalline surfaces such as silicon dioxide or silicon nitride.

In some embodiments of the invention, the gas source for the depositionof epitaxial semiconductor material include a silicon containing gassource, a germanium containing gas source, or a combination thereof. Forexample, an epitaxial Si layer can be deposited from a silicon gassource that is selected from the group consisting of silane, disilane,trisilane, tetrasilane, hexachlorodisilane, tetrachlorosilane,dichlorosilane, trichlorosilane, methylsilane, dimethylsilane,ethylsilane, methyldisilane, dimethyldisilane, hexamethyldisilane andcombinations thereof. An epitaxial germanium layer can be deposited froma germanium gas source that is selected from the group consisting ofgermane, digermane, halogermane, dichlorogermane, trichlorogermane,tetrachlorogermane and combinations thereof. While an epitaxial silicongermanium alloy layer can be formed utilizing a combination of such gassources. Carrier gases like hydrogen, nitrogen, helium and argon can beused.

In some embodiments of the invention, the SiGe sacrificial nanosheetlayers 122, 124, 126 can be about SiGe 20%. The notation “SiGe 20%” isused to indicate that 20% of the SiGe material is Ge and 80% of the SiGematerial is Si. In some embodiments of the invention, the Ge percentagein the SiGe sacrificial nanosheet layers 122, 124, 126 can be any value,including, for example a value within the range from about 20% to about45%.

In embodiments of the invention, the SiGe sacrificial nanosheet layer120 has a Ge percentage that is sufficiently greater than the Gepercentage in the SiGe sacrificial nanosheet layers 122, 124, 126 toprovide etch selectivity between the sacrificial nanosheet layer 120 andthe sacrificial nanosheet layers 122, 124, 126. In some aspects of theinvention, the Ge percentage in the SiGe sacrificial nanosheet layer 120is above about 55%. In some aspects of the invention, the sacrificialnanosheet layers 122, 124, 126 can be SiGe 25%, and the sacrificialnanosheet layer 120 can be at or above about SiGe 55%.

In FIG. 2, known fabrication operations have been used to selectivelyremove the HM 128 (shown in FIG. 1) and the sacrificial nanosheet layer120 (shown in FIG. 1) and replace the layer 120 with a bottom isolationregion 202. In embodiments of the invention, the dielectric materialused to form the bottom isolation region 202 can include silicon oxide,silicon nitride, silicon oxynitride, SiBCN, SiOCN, SiOC, or any suitablecombination of those materials.

Referring still to FIG. 2, known fabrication operations have been usedto, prior to formation of dummy gates 204, deposit a thin layer of gateoxide (not shown separately) over the nanosheet stack 130. In FIG. 2,the dummy gate 204 represents the combination of the thin layer of gateoxide (e.g., SiO₂) and a material (e.g., amorphous silicon (a-Si)) fromwhich the dummy gates 204 are formed.

Referring still to FIG. 2, known fabrication operations (e.g., an RIE)have been used to form dummy gates 204. In embodiments of the invention,the dummy gates 204 can be formed by depositing and planarizing a layerof dummy gate material (not shown) over the gate oxide (not shownseparately from the topmost nanosheet 118). In some embodiments of theinvention, the dummy gate material can be polycrystalline Si. In someembodiments of the invention, the dummy gate material can be amorphousSi (a-Si). After being deposited, the dummy gate material is planarized(e.g., by CMP) to a desired level. Known semiconductor fabricationoperations are used to form patterned/etched hard masks 206 on a topsurface of the planarized dummy gate material. The pattern used to formthe hard masks 206 defines the footprints of the dummy gates 204 and thegate oxide. In embodiments of the invention, the hard masks 206 can beformed from oxide and/or nitride materials. In embodiments of theinvention, the hard masks 206 can be formed by depositing a layer ofhard mask material and patterning then etching the deposited hard masklayer to form the hard masks 206. The dummy gate material is selectivelyetched such that portions of the dummy gate material that are not underthe hard masks 206 are selectively removed, thereby forming the dummygates 204 over the gate oxide and the nanosheet stack 130.

Referring still to FIG. 2, known fabrication operations have been usedto selectively remove the portions of the gate oxide that are not underthe dummy gates 204, and a DHF cleaning has been performed to ensurethat all of the gate oxide that is not under the dummy gates 204 hasbeen removed.

Referring still to FIG. 2, known fabrication operations have been usedto selectively remove the bottommost SiGe 60% sacrificial nanosheetlayer 120 followed by depositing dielectric material used to form offsetgate spacers 208 on sidewalls of the dummy gates 204. The depositeddielectric material also fills in the space that was occupied by theremoved sacrificial nanosheet layer 120, thereby forming a bottomisolation region 202. In embodiments of the invention, the offset gatespacers 208 can be formed by depositing the dielectric material over thenanosheet-base structure 100 then directionally etching (e.g., using anRIE) the dielectric material to form the gate spacers 208. Inembodiments of the invention, the offset gate spacers 208 and the bottomisolation 202 can be formed from any suitable dielectric materialincluding, for example, silicon oxide, silicon nitride, siliconoxynitride, SiBCN, SiOCN, SiOC, or any suitable combination of thosematerials. In some embodiments of the invention, the offset gate spacers208 and/or the bottom isolation region 202 can be a low-k dielectricmaterial.

In FIG. 3, the portions of the nanosheet stack 130 that are not coveredby the gate spacers 208 and the dummy gates 204 are etched, therebyforming multiple instances of the nanosheet stack 130; formingalternating layers of SiGe sacrificial nanosheets 122, 124, 126 and Sinanosheets 114, 116, 118 in each instance of the nanosheet stacks 130;forming S/D trenches 302, 304; providing access to end regions of theSiGe sacrificial nanosheets 122, 124, 126; and providing access to endregions of the Si nanosheets 114, 116, 118. In accordance with aspectsof the invention, each of the S/D trenches 302, 304 includes a cavity312, 314 that extends through the bottom isolation 202 and into thesubstrate 102. Of the 3 (three) nanosheet stacks 130 shown in FIG. 3,the center nanosheet stack 130 will be used to form an active nanosheettransistor. The rightmost and leftmost nanosheet stacks 130 can each bepart of an active or inactive transistor depending on the requirementsof the IC design in which the nanosheet-based structure 100 will beincorporated. Where the rightmost and/or leftmost nanosheet stack 130 ispart of an active transistor, the active transistor formed fromrightmost and/or leftmost nanosheet stack 130 will be in series with thetransistor formed from the center nanosheet stack 130 and will share asource or drain region with the transistor formed from the centernanosheet stack 130. Whether or not the transistors formed from therightmost and leftmost nanosheet stacks 130 are active, the rightmostand leftmost nanosheet stacks 130 define portions of the S/D trenches302, 304 in which S/D regions 1202, 1204 (shown in FIG. 13) will beformed.

In FIG. 4, known semiconductor fabrication processes have been used toform S/D isolation regions 402 in the cavities 312, 314 (shown in FIG.3). In accordance with aspects of the invention, a top surface of eachisolation region 402 is below a top surface of the bottom isolationregion 202 but above a bottom surface of the bottom isolation region202. In some embodiments of the invention, each portion of the bottomisolation region 202 below a nanosheet stack 130 is physically coupledto one or both of the S/D isolation regions 402 as shown in FIG. 4. Insome embodiments of the invention, a vertical depth dimension of thebottom isolation region 202 is less than a vertical depth dimension ofeach the S/D isolation regions 402. In some embodiments of theinvention, a bottom surface of the bottom isolation region 202 issubstantially coplanar with a top surface of the substrate, and a bottomsurface of each isolation region 402 is below the top surface of thesubstrate. In embodiments of the invention, the isolation regions 402can be formed by depositing a dielectric material using a high densityplasma (HDP) anisotropic deposition process followed by an etch back toform the isolation regions 402. The isolation regions 402 can be formedfrom an oxide containing material.

In FIG. 5, known semiconductor fabrication processes have been used topartially remove end regions of the SiGe sacrificial nanosheets 122,124, 126 to form end region or inner spacer cavities 502. In embodimentsof the invention, the end regions of the SiGe sacrificial nanosheets122, 124, 126 can be removed using a so-called “pull-back” process topull the SiGe sacrificial nanosheets 122, 124, 126 back an initialpull-back distance such that the ends of the SiGe sacrificial nanosheets122, 124, 126 now terminate at about an inner edge of the gate spacers208. In embodiments of the invention, the pull-back process leveragesthe fact that the sacrificial nanosheets 122, 124, 126 are formed fromSiGe, which can be selectively etched with respect to the Si nanosheets114, 116, 118 using, for example, a vapor phase hydrogen chloride (HCl)gas isotropic etch process.

In FIG. 6, known semiconductor fabrication operations (e.g., ALD) havebeen used to conformally deposit a layer of inner spacer material 602over the nanosheet-based structure 100. The inner spacer layer 602 canbe silicon nitride, silicoboron carbonitride, silicon carbonitride,silicon carbon oxynitride, or any other type of dielectric material(e.g., a dielectric material having a dielectric constant k of less thanabout 5).

In FIG. 7, known semiconductor fabrication operations (e.g., ananisotropic RIE) can be used to remove the inner spacer material 602from horizontal surfaces of the nanosheet-based structure 100, therebyexposing portions of the top surfaces of the isolation regions 402.

In FIG. 8, known semiconductor fabrication operations have been used toselectively etch the isolation regions 402, thereby forming isolationregion cavities 802. In embodiments of the invention, a dry etch processis used to form the isolation cavities 802, which can result in thetapered sidewalls of the cavities 802. In some embodiments of theinvention, an etch operation can be used to that results insubstantially vertical sidewalls of the cavities 802.

In FIG. 9, known semiconductor device fabrication processes have beenused to form inner spacers 602A. In embodiments of the invention, theinner spacers 602A can be formed using by applying an anisotropic etchback on the inner spacer layer 602 (shown in FIG. 8) to remove excessdielectric material on exposed vertical and horizontal surfaces of thenanosheet-based structure 100, thus leaving the portions of the innerspacer layer 602 that pinched off in the inner spacer cavities 502(shown in FIG. 5), thereby forming the inner spacers 602A.

Referring still to FIG. 9, subsequent to formation of the inner spacers602A, known semiconductor fabrication operations (e.g., ALD) have beenused to conformally deposit over the nanosheet-based structure 100 asemiconductor material layer 902 such that it pinches off in theisolation region cavities 802 (shown in FIG. 8). The semiconductormaterial layer 902 is formed from a material that can act as S/Dformation assistance material in accordance with aspects of theinvention. In embodiments of the invention, the S/D formation assistancematerial is implemented as a nucleation seed material from which the S/Dregions 1202, 1204 (shown in FIG. 13) can be grown (e.g., epitaxiallygrown). In embodiments of the invention, the semiconductor layer 902 isundoped. In embodiments of the invention, the semiconductor layer 902 isundoped poly-silicon. In embodiments of the invention, the semiconductorlayer is formed from Si, SiGe, or III-V semiconductor material, each ofwhich can be undoped.

In FIG. 10, known semiconductor fabrication operations have been used toetch the semiconductor layer 902 (shown in FIG. 9), thereby forming S/Dformation assistance regions, which in accordance with embodiments ofthe invention are implemented as embedded S/D nucleation regions 902A,902B. Additionally, sidewalls of the S/D trench 302 are defined at leastin part by sidewalls of the non-sacrificial nanosheets 114, 116, 118 ofthe leftmost nanosheet stack 130, as well as by sidewalls of thenon-sacrificial nanosheets 114, 116, 118 of the center nanosheet stack130. A bottom surface of the S/D trench 302 is defined at least in partby the top surface of the S/D nucleation region 902A. Similarly,sidewalls of the S/D trench 304 are defined at least in part bysidewalls of the non-sacrificial nanosheets 114, 116, 118 of the centernanosheet stack 130, as well as by sidewalls of the non-sacrificialnanosheets 114, 116, 118 of the rightmost nanosheet stack 130. A bottomsurface of the S/D trench 304 is defined at least in part by the topsurface of the S/D nucleation region 902B.

FIG. 11, known fabrication operations have been used to initiate aprocess for forming the doped S/D regions 1202, 1204 (shown in FIG. 13)in accordance with aspects of the invention. In embodiments of theinvention, the initial stages of the process for forming the doped S/Dregions 1202, 1204 uses an in-situ doped growth process to grow separateindividual S/D sub-regions 1102, 1104, 1106, 1108, 1110, 1112, 1114 inthe S/D trench 302, and to also grow separate individual S/D sub-regions1122, 1124, 1126, 1128, 1130, 1132, 1134 in the S/D trench 304. Inembodiments of the invention, the in-situ doped growth process canutilize gaseous or liquid precursors applied through, for example,vapor-phase epitaxy (VPE), molecular-beam epitaxy (MBE), liquid-phaseepitaxy (LPE), or other suitable processes.

In the S/D trench 302, the S/D sub-regions 1104, 1106, 1108, 1110, 1112,1114 are grown from exposed surfaces at end regions of the nanosheetlayers 114, 116, 118 in the center and leftmost nanosheet stacks 130.Because of the substantially vertical orientation of the exposedsurfaces at end regions of the nanosheet layers 114, 116, 118, the S/Dsub-regions 1104, 1106, 1108, 1110, 1112, 1114 initially grow in asubstantially lateral direction then begin to spread in a substantiallyvertical direction. Also in the S/D trench 302, the sub-region 1102 isgrown from a top surface of the S/D nucleation region 902A. Because ofthe substantially horizontal orientation of the exposed top surface ofthe S/D nucleation region 902A, the S/D sub-region 1102 initially growin a substantially vertical direction then begins to spread in asubstantially lateral direction.

In the S/D trench 304, the S/D sub-regions 1124, 1126, 1128, 1130, 1132,1134 are grown from exposed surfaces at end regions of the nanosheetlayers 114, 116, 118 in the center and rightmost nanosheet stacks 130.Because of the substantially vertical orientation of the exposedsurfaces at end regions of the nanosheet layers 114, 116, 118, the S/Dsub-regions 1124, 1126, 1128, 1130, 1132, 1134 initially grow in asubstantially lateral direction then begin to spread in a substantiallyvertical direction. Also in the S/D trench 304, the sub-region 1122 isgrown from a top surface of the S/D nucleation region 902B. Because ofthe substantially horizontal orientation of the exposed top surface ofthe S/D nucleation region 902B, the S/D sub-region 1122 initially growsin a substantially vertical direction then begins to spread in asubstantially lateral direction.

Referring still to FIG. 11, the in-situ doped growth process is depictedat the beginning of a first “cross-trench” S/D sub-region merger inaccordance with aspects of the invention. As previously noted herein,the terms “cross-trench” refer to an S/D sub-region merger that spansthe merged S/D material across the S/D trench 302 (or 304) in a mannerthat blocks epitaxial gas and/or liquid from flowing to structures belowthe cross-trench merged S/D material. In accordance with aspects of theinvention, the position of the S/D nucleation regions 902A, 902B (i.e.,at a bottom of the S/D trenches 302, 304, respectively) and therelatively large width dimension and/or surface area of the top surfaceof the S/D nucleation regions 902A, 902B ensure that first (or initial)cross-trench S/D sub-region merger occurs in the bottom of each of theS/D trenches 302, 304, and subsequent cross-trench S/D sub-regionmergers occur in a predetermined sequence upward to through each of theS/D trenches 302, 304.

The following description focuses on formation of the S/D region 1202(shown in FIG. 13) in the S/D trench region 302. However, the sameprocesses used to form the S/D region 1202 in the S/D trench 302 areused to form the S/D region 1204 (shown in FIG. 13) in the S/D trench304. Accordingly, in the interest of brevity, the S/D fabricationprocess will not be repeated for the S/D region 1204 except to note thatthe S/D nucleation region 902A corresponds to the S/D nucleation region902B; the S/D sub-region 1102 corresponds to the S/D sub-region 1122;the S/D sub-region 1104 corresponds to the S/D sub-region 1124; the S/Dsub-region 1106 corresponds to the S/D sub-region 1126; the S/Dsub-region 1108 corresponds to the S/D sub-region 1128; the S/Dsub-region 1110 corresponds to the S/D sub-region 1130; the S/Dsub-region 1112 corresponds to the S/D sub-region 1132; and the S/Dsub-region 1114 corresponds to the S/D sub-region 1134.

Referring still to FIG. 11, in embodiments of the invention, theposition of the S/D nucleation region 902A and the relatively largewidth dimension and surface area of the exposed top surface of the S/Dnucleation region 902A ensure that the first cross-trench merger is theS/D sub-region 1102 merging with the S/D sub-regions 1104, 1106. Inaccordance with aspects of the invention, the exposed surface of the S/Dnucleation region 902A is larger than each of the sidewall surfaces atend regions of the non-sacrificial nanosheets. In accordance withaspects of the invention, the exposed surface of the S/D nucleationregion 902A has a lateral width dimension that is larger than a verticalheight (or thickness) dimension of each of the sidewall surfaces at endregions of the non-sacrificial nanosheets 114, 116, 118. In someembodiments of the invention, each of the sidewall surfaces at endregions of the non-sacrificial nanosheets 114, 116, 118 has a verticalheight (or thickness) dimension that is between about 3 nm and about 8nm.

Because the top surface of the S/D nucleation region 902A is normal withrespect to (i.e., positioned at about a 90 degree angle with respect to)the sidewall surfaces at the end regions of the center nanosheet 114 andthe leftmost nanosheet 114, and because the S/D nucleation region 902Ahas a top surface with a relatively large width dimension and/or surfacearea positioned relatively close to the sidewall surfaces at the endregions of the center nanosheet 114 and the leftmost nanosheet 114, thegrowth process applied to the S/D sub-region 1102 and the S/Dsub-regions 1104, 1106 advances these S/D sub-regions (1102, 1104, 1106)toward one another in both lateral (or horizontal) directions and avertical direction. In contrast, the S/D sub-regions 1108, 1110 aregrown from surfaces that are substantially parallel with respect to oneanother, so the S/D sub-regions 1108, 1110 advance toward one another inonly a substantially lateral direction. Similarly, the S/D sub-regions1112, 1114 are grown from surfaces that are substantially parallel withrespect to one another, so the S/D sub-regions 1112, 1114 advance towardone another in only a substantially lateral direction. Hence, theposition and growth pattern of the S/D sub-region 1102 with respect tothe S/D sub-regions 1104, 1106 ensures that the S/D sub-region 1102 willreach (and in effect bridge) the S/D sub-regions 1104, 1106 to form across-trench merged S/D region before the S/D sub-regions 1108, 1110 cancross-trench merge with one another, and before the S/D sub-regions1112, 1114 can cross-trench merge with one another. Accordingly, the S/Dsub-regions 1108, 1110, 1112, 1114 do not block the path for gas and/orliquid to flow to the bottom of the S/D trench 302, which allows thegrowth process applied to the first cross-trench merged S/D sub-regions1102, 1104, 1106 to continue.

FIG. 12 depicts the in-situ doped growth process at the end of a secondcross-trench S/D sub-region merger in accordance with aspects of theinvention. In accordance with aspects of the invention, at thefabrication stage shown in FIG. 11, the first cross-trench merged S/Dsub-regions 1102, 1104, 1106 grow faster than S/D sub-regions that havenot experienced a cross-trench merger, specifically the S/D sub-regions1108, 1110, 1112, 1114. Accordingly, as shown in FIG. 12, a secondcross-trench S/D sub-region merger between the S/D sub-regions 1102,1104, 1106, 1108, 1110 is initiated and completed before the S/Dsub-regions 1112, 1114 can cross-trench merge with one another. Hence,the position and growth pattern of the S/D sub-region 1102 with respectto the S/D sub-regions 1104, 1106, 1108, 1110, as well as the fastergrowth rate of the cross-trench merged S/D sub-regions 1102, 1104, 1106ensures that the cross-trench merged S/D sub-regions 1102, 1104, 1106will reach (and in effect bridge) the S/D sub-regions 1108, 1110 to formthe second cross-trench merged S/D region 1102, 1104, 1106, 1108, 1110before the S/D sub-regions 1112, 1114 can cross-trench merge with oneanother.

As also shown in FIG. 12, in some embodiments of the invention, if thefirst (or initial) cross-trench merged S/D sub-region, which is formedfrom S/D sub-regions 1102, 1104, 1106, forms before the bottom region ofthe S/D trench 302 is completely filled with S/D material, small S/D airgaps 1212 will be formed. In embodiments of the invention, the small S/Dair gaps 1212 function as a marker to indicate that a fabricationoperation in accordance with aspects of the invention has been used toform the S/D regions of a transistor. However, the small S/D air gaps1212 positioned in lower corners of the S/D regions 1202, 1204 do notreduce the solid material volume of the S/D regions 1202, 1204 enough tohave a significant negative impact on the performance of the S/D regions1202, 1204. In some embodiments of the invention, the small air gaps1212 are between about 0% and about 20% of the total volume of each ofthe S/D regions 1202, 1204.

FIG. 13 depicts the in-situ doped growth process at the end of a thirdcross-trench S/D sub-region merger in accordance with aspects of theinvention. In accordance with aspects of the invention, at thefabrication stage shown in FIG. 12, the second cross-trench merged S/Dsub-regions 1102, 1104, 1106, 1108, 1110 grow faster than S/Dsub-regions that have not experienced a cross-trench merger,specifically the S/D sub-regions 1112, 1114. Accordingly, as shown inFIG. 13 the third cross-trench S/D sub-region merger between the S/Dsub-regions 1102, 1104, 1106, 1108, 1110, 1112, 1114 (show in FIG. 12)results in the doped S/D region 1202.

The interface between the S/D regions 1202, 1204 and the non-sacrificialnanosheets 114, 116, 118 create initial p/n junctions. Virtually allsemiconductor transistors are based on the formation of junctions.Junctions are capable of both blocking current and allowing it to flow,depending on an applied bias. Junctions are typically formed by placingtwo semiconductor regions with opposite polarities into contact with oneanother. The most common junction is the p/n junction, which consists ofa contact between a p-type piece of silicon, rich in holes, and ann-type piece of silicon, rich in electrons. N-type and p-type FETs areformed by implanting different types of dopants to selected regions ofthe device to form the necessary junction(s). N-type devices can beformed by implanting arsenic (As) or phosphorous (P), and p-type devicescan be formed by implanting boron (B).

In embodiments of the invention, the doped S/D regions 1202, 1204 can bedoped during deposition (e.g., in-situ doped) by adding dopants such asn-type dopants (e.g., phosphorus or arsenic) or p-type dopants (e.g.,Ga, B, BF₂, or Al) during the above-described methods of forming thedoped S/D regions 1202, 1204. To reduce S/D contact resistance, thedoped S/D regions 1202, 1204 can be highly doped (e.g., doping levels ofabout 1×10²⁰ cm⁻³ to about 1×10²¹ cm⁻³) and can be formed fromSi_(1-x)Ge_(x) having a higher Ge % (e.g., Ge %≥ about 50%). Inembodiments of the invention, the Ge % in the Si_(1-x)Ge_(x) embodimentsof the S/D regions 1202, 1204 can be selected to maximize the dopantsolubility in the Si_(1-x)Ge_(x) S/D regions 1202, 1204. For example, itis generally accepted that a Ge % that can maximize the B solubility inSi_(1-x)Ge_(x) embodiments of the S/D regions 1202, 1204 is a Ge %≥about 65%. In embodiments of the invention, the dopant concentration inthe S/D regions 1202, 1204 can range from about 1×10¹⁹ cm⁻³ to about2×10²¹ cm³, or between about 2×10²⁰ cm⁻³ and about 1×10²¹ cm⁻³.

In embodiments of the invention, a final nanosheet transistor is formedby forming deposing an interlayer dielectric (ILD) (not shown) to fillin remaining open spaces of the nanosheet-based structure 100 (shown inFIG. 13) and stabilize the nanosheet-based structure 100. The structure100 is planarized to a predetermined level that removes the hard masks206. In aspects of the invention, the deposited ILD regions can beformed from a low-k dielectric (e.g., k less than about 4) and/or anultra-low-k (ULK) dielectric (e.g., k less than about 2.5). Areplacement metal gate (RMG) process is applied to the nanosheet-basedstructure 100 to replace the sacrificial nanosheets 122, 124, 126 andthe dummy gates 204 with high-k metal gate (HKMG) structures (notshown). The dummy gates 204 and the gate dielectric (not shown) can beremoved by suitable known etching processes, e.g., RIE or wet removalprocesses. Known semiconductor fabrication operations can be used toremove the SiGe sacrificial nanosheets 122, 124, 126 selective to the Sinon-sacrificial nanosheets 114, 116, 118. In embodiments of theinvention, because the sacrificial nanosheets 122, 124, 126 are formedfrom SiGe, they can be selectively etched with respect to the Sinanosheets 114, 116, 118 using, for example, a vapor phase hydrogenchloride (HCL) gas isotropic etch process.

The HKMG structures (not shown) can be formed using any suitable knownfabrication operations. Each of the HKMG structures includes adielectric layer and a metal gate structure. The HKMG structuressurround the non-sacrificial nanosheets 114, 116, 118 and regulateselectron flow through the non-sacrificial nanosheets 114, 116, 118. Themetal gate structure can include metal liners and work-function metals(WFM). In embodiments of the invention, the WFM can be, for example, TiNor TaN, and the metal gate structure can be aluminum or tungsten. Thedielectric layer can include interfacial layers (IL) and high-kdielectric layers. In some embodiments of the invention, the high-kdielectric layers can modify the work function of the WFM. The high-kdielectric layer can be made of, for example, silicon oxide, siliconnitride, silicon oxynitride, boron nitride, high-k materials, or anycombination of these materials. Examples of high-k materials include butare not limited to metal oxides such as hafnium oxide, hafnium siliconoxide, hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminumoxide, zirconium oxide, zirconium silicon oxide, zirconium siliconoxynitride, tantalum oxide, titanium oxide, barium strontium titaniumoxide, barium titanium oxide, strontium titanium oxide, yttrium oxide,aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate. Thehigh-k materials can further include dopants such as lanthanum andaluminum.

The methods and resulting structures described herein can be used in thefabrication of IC chips. The resulting IC chips can be distributed bythe fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includes ICchips, ranging from toys and other low-end applications to advancedcomputer products having a display, a keyboard or other input device,and a central processor.

Various embodiments of the present invention are described herein withreference to the related drawings. Alternative embodiments can bedevised without departing from the scope of this invention. Althoughvarious connections and positional relationships (e.g., over, below,adjacent to, etc.) are set forth between elements in the detaileddescription and in the drawings, persons skilled in the art willrecognize that many of the positional relationships described herein areorientation-independent when the described functionality is maintainedeven though the orientation is changed. These connections and/orpositional relationships, unless specified otherwise, can be direct orindirect, and the present invention is not intended to be limiting inthis respect. Similarly, the term “coupled” and variations thereofdescribes having a communications path between two elements and does notimply a direct connection between the elements with no interveningelements/connections between them. All of these variations areconsidered a part of the specification. Accordingly, a coupling ofentities can refer to either a direct or an indirect coupling, and apositional relationship between entities can be a direct or indirectpositional relationship. As an example of an indirect positionalrelationship, references in the present description to forming layer “A”over layer “B” include situations in which one or more intermediatelayers (e.g., layer “C”) is between layer “A” and layer “B” as long asthe relevant characteristics and functionalities of layer “A” and layer“B” are not substantially changed by the intermediate layer(s).

The following definitions and abbreviations are to be used for theinterpretation of the claims and the specification. As used herein, theterms “comprises,” “comprising,” “includes,” “including,” “has,”“having,” “contains” or “containing,” or any other variation thereof,are intended to cover a non-exclusive inclusion. For example, acomposition, a mixture, process, method, article, or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements but can include other elements not expressly listed or inherentto such composition, mixture, process, method, article, or apparatus.

The term “exemplary” is used herein to mean “serving as an example,instance or illustration.” Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs. The terms “at least one”and “one or more” are understood to include any integer number greaterthan or equal to one, i.e. one, two, three, four, etc. The terms “aplurality” are understood to include any integer number greater than orequal to two, i.e. two, three, four, five, etc. The term “connection”can include an indirect “connection” and a direct “connection.”

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedcan include a particular feature, structure, or characteristic, butevery embodiment may or may not include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper,” “lower,”“right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” andderivatives thereof shall relate to the described structures andmethods, as oriented in the drawing figures. The terms “overlying,”“atop,” “on top,” “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements such as an interfacestructure can be present between the first element and the secondelement. The term “direct contact” means that a first element, such as afirst structure, and a second element, such as a second structure, areconnected without any intermediary conducting, insulating orsemiconductor layers at the interface of the two elements.

Spatially relative terms, e.g., “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

The terms “about,” “substantially,” “approximately,” and variationsthereof, are intended to include the degree of error associated withmeasurement of the particular quantity based upon the equipmentavailable at the time of filing the application. For example, “about”can include a range of ±8% or 5%, or 2% of a given value.

The phrase “selective to,” such as, for example, “a first elementselective to a second element,” means that the first element can beetched and the second element can act as an etch stop.

The term “conformal” (e.g., a conformal layer) means that the thicknessof the layer is substantially the same on all surfaces, or that thethickness variation is less than 15% of the nominal thickness of thelayer.

References in the specification to terms such as “vertical”,“horizontal”, “lateral,” etc. are made by way of example, and not by wayof limitation, to establish a frame of reference. Terms such as“horizontal” and “lateral” refer to a direction in a plane parallel to atop surface of a semiconductor substrate, regardless of its actualthree-dimensional spatial orientation. Terms such as “vertical” and“normal” refer to a direction perpendicular to the “horizontal” and“lateral” direction.

As previously noted herein, for the sake of brevity, conventionaltechniques related to semiconductor device and IC fabrication may or maynot be described in detail herein. By way of background, however, a moregeneral description of the semiconductor device fabrication processesthat can be utilized in implementing one or more embodiments of thepresent invention will now be provided. Although specific fabricationoperations used in implementing one or more embodiments of the presentinvention can be individually known, the described combination ofoperations and/or resulting structures of the present invention areunique. Thus, the unique combination of the operations described inconnection with the fabrication of a semiconductor device according tothe present invention utilize a variety of individually known physicaland chemical processes performed on a semiconductor (e.g., silicon)substrate, some of which are described in the immediately followingparagraphs.

In general, the various processes used to form a micro-chip that will bepackaged into an IC fall into four general categories, namely, filmdeposition, removal/etching, semiconductor doping andpatterning/lithography. Deposition is any process that grows, coats, orotherwise transfers a material onto the wafer. Available technologiesinclude physical vapor deposition (PVD), chemical vapor deposition(CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE)and more recently, atomic layer deposition (ALD) among others.Removal/etching is any process that removes material from the wafer.Examples include etch processes (either wet or dry), chemical-mechanicalplanarization (CMP), and the like. Reactive ion etching (RIE), forexample, is a type of dry etching that uses chemically reactive plasmato remove a material, such as a masked pattern of semiconductormaterial, by exposing the material to a bombardment of ions thatdislodge portions of the material from the exposed surface. The plasmais typically generated under low pressure (vacuum) by an electromagneticfield. Semiconductor doping is the modification of electrical propertiesby doping, for example, transistor sources and drains, generally bydiffusion and/or by ion implantation. These doping processes arefollowed by furnace annealing or by rapid thermal annealing (RTA).Annealing serves to activate the implanted dopants. Films of bothconductors (e.g., poly-silicon, aluminum, copper, etc.) and insulators(e.g., various forms of silicon dioxide, silicon nitride, etc.) are usedto connect and isolate transistors and their components. Selectivedoping of various regions of the semiconductor substrate allows theconductivity of the substrate to be changed with the application ofvoltage. By creating structures of these various components, millions oftransistors can be built and wired together to form the complexcircuitry of a modern microelectronic device. Semiconductor lithographyis the formation of three-dimensional relief images or patterns on thesemiconductor substrate for subsequent transfer of the pattern to thesubstrate. In semiconductor lithography, the patterns are formed by alight sensitive polymer called a photo-resist. To build the complexstructures that make up a transistor and the many wires that connect themillions of transistors of a circuit, lithography and etch patterntransfer steps are repeated multiple times. Each pattern being printedon the wafer is aligned to the previously formed patterns and slowly theconductors, insulators and selectively doped regions are built up toform the final device.

The flowchart and diagrams in the Figures illustrate possibleimplementations of fabrication and/or operation methods according tovarious embodiments of the present invention. Variousfunctions/operations of the method are represented in the flow diagramby blocks. In some alternative implementations, the functions noted inthe blocks can occur out of the order noted in the Figures. For example,two blocks shown in succession can, in fact, be executed substantiallyconcurrently, or the blocks can sometimes be executed in the reverseorder, depending upon the functionality involved.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments described. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdescribed herein.

What is claimed is:
 1. A method of performing fabrication operations toform a transistor, wherein the fabrication operations include: forming asource or drain (S/D) region having an S/D formation assistance region;wherein the S/D formation assistance region comprises a top surface,sidewalls, and a bottom surface; wherein the S/D formation assistanceregion is at least partially within a portion of a substrate; andforming an S/D isolation region around the sidewalls and the bottomsurface of the S/D formation assistance region; wherein the S/Disolation region is configured to electrically isolate the S/D formationassistance region from the substrate.
 2. The method of claim 1, whereinthe fabrication operations further include: forming over the substrate afirst nanosheet having a first nanosheet end region, wherein a sidewallsurface of the first nanosheet end region defines a first portion of asource or drain (S/D) trench; and forming over the first nanosheet asecond nanosheet having a second nanosheet end region, wherein asidewall surface of the second nanosheet end region defines a secondportion of the S/D trench, wherein the top surface of the S/D formationassistance region defines a bottom surface of the S/D trench; whereinforming the S/D region comprises: growing a first portion of the S/Dregion from the sidewall surface of the first nanosheet end region;growing a second portion of the S/D region from the sidewall surface ofthe second nanosheet end region; and growing a third portion of the S/Dregion from the top surface of the S/D formation assistance region;wherein the third portion merges with the second portion to form a firstmerged portion of the S/D region; and wherein, subsequent to formationof the first merged portion, the first merged portion merges with thefirst portion to form a second merged portion of the S/D region.
 3. Themethod of claim 1, wherein: the top surface of the S/D formationassistance region is substantially horizontal; and a portion of the S/Dregion is grown from the substantially horizontal top surface of the S/Dformation assistance region.
 4. The method of claim 1, wherein formingthe S/D region having the S/D formation assistance region furthercomprises: forming an S/D formation assistance region trench in thesubstrate; forming the S/D isolation region in the S/D formationassistance region trench; and depositing the S/D formation assistanceregion within an opening in the S/D isolation region such that the S/Disolation region electrically isolates the S/D formation assistanceregion from the substrate.
 5. The method of claim 2 further comprisingforming a bottom isolation region positioned between the first nanosheetand the substrate.
 6. The method of claim 5, wherein the portion of thesubstrate comprises the S/D isolation region configured to electricallyisolate the S/D formation assistance region from the substrate.
 7. Themethod of claim 6, wherein the bottom isolation region is physicallycoupled to the S/D isolation region.
 8. The method of claim 7, wherein avertical depth dimension of the bottom isolation region is less than avertical depth dimension of the S/D isolation region.
 9. A method ofperforming fabrication operations to form a transistor, wherein thefabrication operations include: forming over a substrate a firstnanosheet having a first nanosheet end region, wherein a sidewallsurface of the first nanosheet end region defines a first portion of asource or drain (S/D) trench; forming over the first nanosheet a secondnanosheet having a second nanosheet end region, wherein a sidewallsurface of the second nanosheet end region defines a second portion ofthe S/D trench; forming over the substrate a third nanosheet having athird nanosheet end region, wherein a sidewall surface of the thirdnanosheet end region defines third portion of the S/D trench; formingover the third nanosheet a fourth nanosheet having a fourth nanosheetend region, wherein a sidewall surface of the fourth nanosheet endregion defines a fourth portion of the S/D trench; forming an S/Dformation assistance region within a portion of the substrate, wherein atop surface of the S/D formation assistance region defines a bottomsurface of the S/D trench; and forming an S/D region in the S/D trench;wherein forming the S/D region comprises: growing a first portion of theS/D region from the sidewall surface of the first nanosheet end region;growing a second portion of the S/D region from the sidewall surface ofthe second nanosheet end region; growing a third portion of the S/Dregion from the sidewall surface of the third nanosheet end region;growing a fourth portion of the S/D region from the sidewall surface ofthe fourth nanosheet end region; and growing a fifth portion of the S/Dregion from the top surface of the S/D formation assistance region;wherein, prior to the first portion merging with the third portion, thefifth portion merges with the second portion and the third portion toform a first merged portion of the S/D region.
 10. The method of claim9, wherein forming the S/D formation assistance region comprises forminga S/D formation assistance region trench in the substrate.
 11. Themethod of claim 10, wherein forming the S/D formation assistance regionfurther comprises forming an S/D isolation region in the S/D formationassistance region trench.
 12. The method of claim 11, wherein formingthe S/D formation assistance region further comprises depositing the S/Dformation assistance region within an opening in the S/D isolationregion such that the S/D isolation region electrically isolates the S/Dformation assistance region from the substrate.
 13. The method of claim9 further comprising: forming a first bottom isolation region positionedbetween the first nanosheet and the substrate; and forming a secondbottom isolation region positioned between the third nanosheet and thesubstrate.
 14. The method of claim 13, wherein the portion of thesubstrate comprises an S/D isolation region configured to electricallyisolate the S/D formation assistance region from the substrate.
 15. Themethod of claim 14, wherein: the first bottom isolation region isphysically coupled to the S/D isolation region; and the second bottomisolation region is physically coupled to the S/D isolation region. 16.The method of claim 15, wherein a vertical depth dimension of the firstbottom isolation region is less than a vertical depth dimension of theS/D isolation region.
 17. The method of claim 15, wherein a verticaldepth dimension of the second bottom isolation region is less than avertical depth dimension of the S/D isolation region.